A flange coupling is widely used for connecting end portions of ducts. The flange coupling is a member configured to couple flanges provided at the end portions of the ducts and butted to each other by tightening with tightening members such as bolts and nuts.
The flange coupling as described above is generally provided with a seal ring such as an O-ring clamped between the flanges so as to prevent fluid passing in the interior of the ducts to be connected from leaking to the outside through between the butted flanges.
However, the seal ring is generally formed of rubber or resin, deterioration over time may occur, and hence regular replacement work is required and the replacement work in a duct apparatus or piping having a number of connecting portions is troublesome, so that lowering of efficiency of operation of the duct apparatus may be resulted. Also, when the duct apparatus is an apparatus which handles food, for example, there is a possibility that the smell of the O-ring is transferred to the food, or chips of the deteriorated O-ring are mixed into the food. In addition, the O-ring has low pressure-resistance and hence is not suitable for being used in the duct apparatus which handles high-pressure fluid and, in particular, it cannot be used in the duct apparatus which handles ultrahigh pressure fluid at tens of MPa such as liquefied hydrogen, which is increasingly needed in recent years. Furthermore, although the O-ring is set by being fitted to an annular grooves formed on surfaces of opposed flanges, in the case of the duct which handles food or the like, the food may get into the grooves, which is not preferable from the aspect of good hygiene and, in addition, there is also a problem in that disassembly and washing of the coupling become difficult.
In contrast, there is developed a metal seal ring, which is a seal ring formed of metal such as stainless or the like (for example, Patent Documents 1 and 2). Although the metal seal ring as described above has an advantage in being less deteriorated than those formed of rubber or resin, improvement in sealing capability and the like is required.
In contrast, a technology to connect the ducts without using packing has been developed (Patent Document 4). This technology achieves sealing by providing an annular projection having a tapered outer peripheral surface on an end flange of one of ducts to be joined, press-fitting the annular projection to a duct hole of a partner duct, and pressing the outer peripheral surface of the annular projection against an end edge of the duct hole. This technology allows the ducts to be coupled without the seal ring such as the O-ring, and is an effective member which can solve the above-described problem occurring when the seal ring is used.